Kov., 19G1
PHOTOCHEMISTRY OF IODIDE Iox
is slowly increased. The latter results in a decrease of the slope of the differential temperature curve. After the slope has been reduced to zero, the measurements of time (ts t l ) and temperature ( T z - TI) and (To - T i ) are made. Although the temperature measurements may be made with the aid of a potentiometer, no substantial loss in accuracy was found when using the recorder. The method was tested by determining the known heats of fusion of KNO,, AgNO3, il’aNOc and KzCr207. Considerable difficulty was encountered when attempting to find the heating rates a t which the thermal gradient remained constant. The difficulty is due to the thermal inertia of
IN
AQUEOUS SOLUTION
1937
the system. Although the heating rate can be changed rapidly, the results of the change can be measured only after a time lag during which the system tends to reach a new state of equilibrium. At the present time, research efforts are directed toward finding a more convenient laboratory technique which either would simplify the procedure for finding the required heating rate or would obviate the need for finding the rate. No limits of accuracy can yet be given. The accuracy is of coursealso a function of the quality of the amplifying and recording equipment and the accuracy of instrument calibration and sample weighing.
THE PHOTOCHEMISTRY OF IODIDE ION I N AQUEOUS SOLUTION1 BY E. HAY ON^ Chemistry Department, Brookhawen National Laboratory, Upton, Long Island, New York Received January SO, 1961
The quantum yield for the formation of IZ on exposure of air-free aqueous solutions of iodide to 2537 A. light is dependent upon tile concentration of iodide and hydrogen ions, as was observed by other workers. I n dilute solutions of iodide at pH > 3, +I* is almost zero. Addition of acceptors for electrons or hydrogen atoms, such as H20zand KNOs, brings about an iodide-induced decomposition of these scavengers in neutral and alkaline solutions. This indicates, contrary t o the postulates of Farkas and Farkas and of Platzman and Franck, that hydrogen ions per se are not essential in the primary photochemical process. The dependence of +I* upon [H+] may be due to the formation of Hz+ as proposed by Rigg and hv
+H
Weiss, or to the scavenging of electrons by the hydrogen ions in solution (I- HzO) +(I- HzO)*and (I-HzO)* I H H:O.
+ +
A number of mechanisms have been postulated for the primary photochemicsl processes occurring in the oxidation of iodide ions in aqueous solutions. As early as 1928 Franck and Scheibe3characterized the absorption bands of iodides as “electron affinity spectra,” and in 1931 Franck and Haber4 proposed that the absorption of energy by iodide ions resulted in the transfer of the electron from the anion t o a water molecule on the hydration sphere. Farkas and F a r k a ~ in , ~ order to explain
+
+
of the quantum yield upon H + and I- concentrations by proposing the formation of Hz+ hv (I- H20) --+ (I- HzO)* (I- HzO)* +I H OHH H + --+ Hz+ H2++I-+I+Hz
+
+ +
More recently Platzman and Franck7,*proposed a model for the excited complex in which the electron transfer process was based on the rate of hv collision between ionic species. To explain the (I- Hz0) --+-( I HzO-) +I H OHdecline in the quantum yield of iodine with dethe dependence of the photooxidation of iodides creasing hydrogen ion concentration, they presumed upon the hydrogen ion concentration, suggested the photolytic process to result from a collision that the transfer of the electron occurred from the of a hydrogen ion with the entity formed by the hydration sphere to a hydrogen ion in solution. absorption act.7 Smith and Symonds,g in trying to explain the dependence of the absorption band hY (I- H20) +(I HzO-) of iodide on temperature and the nature of the ( I &0-) + (I- H20) heat solvent used, have suggested another model based ( I HZO-) + H + +I H Hz0 on a modification of that proposed by Plataman Rigg and Weiss6 investigated the photochemistry and Franck.’J The experiments carried out in this work indicate of iodide solutions over a wide range of iodide and hydrogen ion concentrations, and observed that that hydrogen ions are not essential in the primary the quantum yield was dependent upon both H + photochemical process resulting in the oxidation and I- concentrations. Since the Farkas and of iodide ions, since such an oxidation was obtained Farkas mechanism excluded a dependence of the in oxygen-free neutral or alkaline solutions of iodide quantum yield on iodide concentration, Rigg and in the presence of solutes which can themselves Weiss postulated the formation of H and I atoms scavenge electrons or hydrogen atoms. The hyin aqueous solution, and explained the dependence drogen ion can, however, play the same role as the other solutes used insofar as it too can scavenge (1) Researon performed under the auspices of the U. 8. Atomic electrons or hydrogen atoms, but with much lower Energy Commission. efficiency. Recently, after the completion of (2) Department of Physical Chemistry, Cambridge University,
+ +
+ + +
England. ( 8 ) J. Franck and G. Srheibp. Z. physik. Chem., A159, 22 (1928). ( 4 ) J. Franck and I?. IIaber, S. E . preuss. Akad. Wvisrr. (Phys. M d h . ) , 250 (1931). ( 5 ) A. Farkas and I,. Farkas, Trans. Faraday Soc.. 54, 1113 (1938). (0) T. Rigg and J. Weiss, J. Chem. Soc., 4198 (1952).
(7) R. Platzman and .J. Franck, Research Council of Israel, Bperial Publication No. 1. Jerusalem, 1952, p. 21. ( 8 ) R. Platzman and J. Franck, Z . Physzk, 158, 411 (1954). (9) hl. Smith and M. C. R. Symonds. Trans. Faraday Soc., 64,346 ( 1958).
E. HAYON
1938
Vol. 65
Results
1
10
50 TIME
60 70 IRR.(MINI
~
l
80
90
I
100
l
110
120
Fig. 1.-Iodine produced by illumination of potassium iodide solutions, N? atmosphere unless stated otherwise: 0, 10-3 KI, p~ 5.7; 0 , 10-1 M , PH 6.2; O, 10-1 M , pH 9.2; 111, 1 I ! , pH 6.5; A, 1.0 M,air-saturated; A, 1.0 M , air-saturated, lower light intensity.
x
this work, Dainton and Sills*O briefly reported experiments on the ultraviolet irradiation of aqueous solutions of iodide in the presence of nitrous oxide which yielded similar results.
Experimental A low pressure mercury resonance lamp Hanovia SC2537 was used, run from a 6000 volt transformer a t 50 ma.
in all cases except a few runs at lower intensity in which 20 ma. current was used. The radiation cells were cylindrical (of the tvpe described by Saldick and .411en11), 5 X 2.5 cm. diam., made of quartz glass with a Vycor window on the side incident to the light, such that all the 1849 A. mercury resonance line was absorbed by the Vycor glass. The volumes used were about 22 to 25 cc. and the solutions were stirred magnetically. A water-lubricated stopcock and a quartz frit were sealed on a t the bottom of the cell. The 1i;ght from thc lamp first passed through a Vycor condenser of 5 em. internal diameter containing triply distilled water, before rearhing the radiation cells. All experiments were done under conditions of complete absorption of the incident light by the iodide. The light input was measured by means of the ferric ovalate actinometer12 (+ = 1.2) under conditions identical to the experiments RTith iodide. The intensity a t which almost all the work was done was 3.16 X 10-5 einsteins/l.-min.; the lower intensity used was 7.9 X 10--6 einsteins/l.-min. The water used in preparing the solutions was triply distilled from acid dichromate, alkaline permanganate and a final di~tillation.1~ Stock solutions of 2 X 10-3 M potassium iodide (Baker Analyzed reagent) were prepared weekly; solutions of iodide above 2 X 1 0 - 3 , M were prepared just previous to radiation. The solutions were deaerated by bubbling prepurified nitrogen for about 20 min. through the quartz frit, while the solution was magnetically stirred. A small quartz stopper above displaced some of the solution so as to leave no air-space above the liquid. Iodine in the irradiated solutions wxs measured directly as 13- at 350 mp, whose concentration in 0.1 -If I- was 38.9 p M per optical density unit. The sum of the yields of hydrogen pwoxide and iodine was mzasured using Ghormley 's iodide reagentI4 which contains molybdate as catalyst. Blanks for the unirradiated solutions as well as for the iodide reagent vere done simultaneously in all cases. Sulfuric acid or sodium hydroxide was used to adjust the pH. Nitrite was determined by the method of Shin,I5a and the procedure and extinction coefficient used were those of Schwara and Allen.1sb (IO) F. t;. Dainton and 6. A. Sills, Suture, 186, 879 (1960). (11) J. Saldick and A . 0. illlen, J . Chem. P h y s . , 22, 438 (1954). (12) C . (3. Hatcharif and C . A. Parker, Proc. Roy. SOC.(London), 2388, 518 I 19,561. (13) E. 12. .Io21nson and A. 0. .Wen, J . Am. Chem. SOC.,74, 4147 (1952). (14) C. J . Hochanadel, J . P h y s . Chem., 66, 587 (1952). (15) (a) M. B. Shin, Ind. Eng. Chem., Anal. Ed., 13, 33 (1941); b ) H. 8. Schwarz and A. 0. Allen, J. Am. Chem. SOC.,77, 1324 195 5).
I n the photochemistry of A4 neutral airfree solutions of iodide only a very small amount of iodine, and equivalent hydrogen as recently has been measured by Edgecombe and Norrish,I6 is formed, as shown in Fig. 1. Increasing the iodide concentration produces an increase in the rate of formation of iodine both initially and at Ionger times. Addition of hydrogen peroxide to neutral air-free solutions of iodide brings about photoinduced changes in the concentration of the added solute (Fig. 2), and addition of nitrate to neutral air-free solutions of iodide increases the quantum yield for the photochemical oxidation of iodide (Fig. 3). The photochemical changes taking place are dependent on the nature as well as the concentration of the added substance. The amounts of HzOz and NOa- added are relatively small and they absorb less than 2% of the incident light. I n the absence of iodide the concentration of the added solutes remains unchanged on irradiation, under otherwise identical experimental conditions. When HzOzis added to neutral dilute iodide solutions, the amount of iodine formed is still very small (< 1 p&l) but hydrogen peroxide is decomposed, Fig. 2, with a yield independent of pH between 5.7 and 10.5, but dependent upon the concentration of initially added HzOz (in l o p 3 AI KI, ~ ( - H ~ o ? )= 0.11 and 0.05 for 222 and 54 pilf HzOzsolutions, respectively). Owing to the thermal reaction between I- and H202! which takes place even in neutral solution, the decomposition of HzOzcould not be investigated a t high I- concentration. The addition of potassium nitrate to air-free neutral solutions of iodide results on illumination in the formation of substantial amounts of iodine and nitrite (Fig. 3). However, when HzS04 is added to the mixture to bring the pH to 2.0! very little nitrite or iodide is formed. ?\To reduction of nitrate takes place in the absence of iodide, indicating that the reduction of nitrate is photoinduced by the iodide. Figure 4 shows the dependence of the yield of iodine upon [H+]and [I-].
Discussion The processes resulting from the absorption of light by iodide ions, present in neutral air-free aqueous solutions, are hu
I- HzO --+ ( I - HzO)* (I- H20)*--+I- HzO heat (I- H,O)* --+ I H20(I- HZO)* +I H OHH I ---+ H + II+I---tI, H+H--+Hz H 1 2 ---+I I-+ H-
+
+
+ + + + +
+
The effect of iodide coiicentration upon the formation of iodine (+I, = 0.008 and 0.016 for 0.1 and 1.0 dl KI, respectively) from air-free neutral and Roy. SOC. (16) F. H. C . Edgecornbe and R. G. Tv. Norrish, PTOC. (London),2638, 154 (1959).
Nov., 1961
PHOTOCHEMISTRY OF IODIDE IONIN AQUEOUS SOLUTIOX
1939
- - I
rnL
/
TIME IRR (MINI. of KI (2 x 10-3 M ) in presence of H202, N,atmosphere: 0, 200 ptM/1. HzO,, pH 5.7; 0,110 ,uM/l., pH 5.7; A, 47 pM/l., pH 5.7; A, 105 pM/1., pH 9.4; e, 95 ,uM/l., pH 10.6.
Fig. Z.-Photochemistry
alkaline iodide solutions (Fig. 1) can be explained as being a result of the formation of IzI
+ I- +1 2 -
(8)
an unstable species whose absorption spectrum has been observed in flash photolysis of iodide solutions by Grossweiner and Matheson17 and Edgecombe and Norrish,16 and is believed to be the dihalide. The rate of disappearance of 1 2 - was found17to be independent of pH but to increase with a decrease in the iodide concentration. The effect of iodide concentration (Fig. 1) in the steady irradiation may be explained if one assumes that the rate constant for reaction 4 is greater than that for reaction 9 since the rate of reaction 5 is greater than that of (10)17 €1 Iz-
+ +H + + 21+ I,- +Is- + I12-
(9) (10)
The minimum value for the rate constant of reaction 10 was found17 to be 1.2 X lo4 and at the lowest [I-] used in these experiments, M, most I atoms will be present as Iz-. It is also possible that the reduction of the trihalide 1,- is less favorable than that of 1 2 . No iodine (< 1pi7.1) is formed on addition of HzOz to neutral or alkaline dilute solutions of iodide, but hydrogen peroxide is decomposed (Fig. 2). I n the absence of iodide the concentration of HzOz remains unchanged on irradiation; furthermore Hunt and Taube18 showed that the photodecomposition of aqueous solutions of hydrogen peroxide is unaffected by the addition of halides. It follows therefore that the photodecomposition of HzOs observed in neutral and alkaline air-free solutions (17) L. I. Ckrossrveiner and Jf. S. Matheson, J . P h w . Chem., 61, 1089 (1957). (18) J. P. Hunt and H. Taube, J . Am. Chenl. Soc., 74, 5999 (1952).
u
'0
IO
20
30
40
50
60
70
TIME IRR.IMIN)
Fig. 3.-Forniation of iodine and nitrite on illumination of neutral solutions of potassium iodide and potassium nitrate: 0, I g and 0,S O - found in a solution of M KI and M KN03; 0 , 1 2 and . , XO2- in M KI and A f KXO&
of iodide must be induced by the iodide which absorbs more than 98% of the incident light. The following reactions take place H (or HzO-)
+ H20s--+H20 + OH (+OH-) OH + I - -+ OH- + I
(11) (12)
(19) (a) AT. G. Evans. N. S. Hush and N. Uri, Quart. R e m , 6, 186 (1952); E. J. Hart and C. B. Senvar, Second Intl. Conf. Peaceful Uses of Atomic Energy, Genei-a (1958); (b) A. 0. Allen and H. 4. Schwarz, J . Chem. Phys., 29, 30 (1958). (20) E. Hason and A. 0. .411en, to be published.
E.
19-10
1-01. 65
HAYON
+ NOz- +NO + O H + NOz- +21- + KO2 NO + NOz + Hz0 +2HNOz H
12-
(18) (19) (20)
12-
!
0
IO
20
30
I
40
50
60
I
I
I
I
I
I
70
80
90
100
110
120
TIME IRR. (MIN).
Fig. 4.-Iodine produced by illumination of KI in acid solutions, NZatmosphere: 0, 10-3 M KI, pH 1.2; 0,lo-* M , pH 2.0; A, 10-l M , pH 2.9; 0, 10-l M , pH 2.2; A, M , pH 1.4.
Iz-
+ HzOz +H + + 21- + HOz + HOz -+ Hi02 + Oz H + +HOz H20- + Oz +02-+ HzO
HOz
0 2
(13) (14) (15) (150
The competition between reactions 11 and 15 or 15’ accounts for the departure from linearity of (-HzOz) as a function of time of illumination Fig. 2). It was shownlgbin radiation chemistry that [k(l5)/k(ll)] for neutral solutions is 1.85 and the results in Fig. 2 are within i10010,in good agreement with this value. It is not possible from the above experiments to distinguish categorically whether the entities which reduce H2Oz are H atoms or H20-. The effect of HzOz, however, is to reduce the back reactions which take place in the absence of added scavengers, reactions 4-7. M KI in the presence of 60 On irradiation of p M H202 a t pH 2.9 to 4.3, iodine is formed and an equivalent amount of Hz02 decomposed. These results are in agreement with the finding in the radiation chemistry20 of I-/H202 solutions that reaction 13 does not take place, ie., is reversed in acid solution. At pH below 2.9, 2-3 pM I2 only is formed and the concentration of HzO2 remains unchanged. This shows that the H species formed at low pH in the photochemistry of iodide (reaction 3) are relatively unreactive toward hydrogen peroxide, as was observed in the radiation chemistry under similar ~0nditions.l~It appears therefore that the reducing species formed in the radiation chemistry of water are similar to those formed in the photochemistry of aqueous solutions of iodide. The addition of KO3- ions to air-free neutral solutions of iodide results in the formation of 1 2 and reduction of nitrate to nitrite (Fig. 3). Initially the mechanism of formation of I2 and KO,is probably
+ NO,- +NOz + OH12- + Is- + I2N02 + HzO --+HNOZ + HNOB H
12-
--3
(16) (10) (li)
Secondary reactions, however, take place and the ensuing steps are not clear. The irradiation of 10-3 M K I and 10-4 M KNO2 in neutral air-free solution results in no net oxidation of nitrite and no formation of iodine. This is explained as a result of the reaction of hydrogen and iodine atoms with nitrite, followed by reaction 20
Reactions 16-20 already have been The dependence of the quantum yield of iodine upon hydrogen ion concentration (Fig. 4) may be due to one or both of the following possibilities: (i) A result of formation of hydrogen molecule ions, H2+, as postulated by Rigg and which would account for the increase in 61%with I- and H + concentrations H Hz+
+ H + +Hz+
+ I-
(21) (22)
+ Hz + I
(ii) The dependence on hydrogen ion concentration may be similar to that of hydrogen peroxide or nitrate in the photochemistry of neutral iodide solutions, ie., hydrogen ions react with the H atom precursor, the hydrated electrons, in the primary photochemical process (H20H+ H H20) and no H2+ formation need be considered. The effect of the [I-] in both cases would be to stabilize the I atom as an 12-, and reduce the back reactions 4 and 5 . It does not seem at present possible to differentiate between the two alternatives. In the photochemistry of neutral air-saturated M solutions of iodide no H202 is formed and only traces (
